Most telecommuting (TC) studies focus on travel impacts and do not consider changes in time spent on non-travel activities (e.g. 'leisure') and the energy impacts of these changes. We demonstrate a time-use approach to assess interrelations between changes in commuting time and time spent on travel and non-travel activities and associated energy impacts. Time-use data analysis shows that spending less time on commuting is associated with more time spent on 'sleep', 'leisure', 'personal, household and family care', 'private travel' and 'eating and drinking'. Substituting car commuting with 'sleep', 'eating and drinking', common 'leisure' and 'personal, household and family care' activities is likely to reduce energy requirements as these are associated with less energy requirements than car commuting. This is different for 'private travel', 'meal preparation at home', and energy-intensive or out-of-home 'leisure' activities, which are associated with relatively high energy requirements. The commute modal split is a key variable in energy impacts of TC, because transport modes differ in their energy requirements. While car commuters can realize high energy savings through TC, for people who usually bike or walk to work, direct energy savings through reduced commuting are zero. Thus, any additional energy impact due to substitute activities, increases net direct energy requirements. Future research should further investigate the relationship between TC and time spent on (non-)travel activities and the marginal energy requirements of these activities. If so, the time-use approach can become key for assessing energy impacts of TC and other applications which impact individual time allocation.